1. Field of the Invention
The present disclosure relates to a nuclear fuel rod for fast reactors that includes a metallic fuel slug coated with a protective coating layer and a fabrication method thereof.
2. Description of the Related Art
The present invention relates to a process for improving the performance of nuclear fuel for reactors, and more particularly, to a technique that stabilizes components of a metallic fuel slug and fission products or impurities through the stabilization of surfaces of the metallic fuel slug and metallic fuel powder by a surface treatment.
Nuclear fuel in fast reactors is designed in various types, such as a plate type, a pellet type, and a rod type, and a fissionable material that undergoes a nuclear reaction is included in a nuclear fuel rod. The fissionable material is sealed by a container, which is not reactive due to its good compatibility with a coolant and has good heat transfer characteristics, i.e. a cladding tube. The nuclear fuel rods being maintained at a constant spacing are assembled in the form of a fuel assembly and the assembly is charged into a nuclear reactor. In this case, the cladding tube surrounding the fuel must prevent chemical interactions between the fissionable material and the coolant by blocking a direct contact therebetween and must prevent the leakage of fission products. In addition, in fast reactors using metallic nuclear fuel, it is highly advantageous in terms of the safety and economic efficiency of nuclear fuel to also inhibit interactions between the cladding tube and the fissionable material.
In particular, in fast reactors using metallic fuel, a phenomenon occurs, in which a melting temperature of a metallic fuel slug decreases or the strength of a cladding tube decreases by the interpenetration between components (uranium (U), plutonium (Pu), thorium (Th), minor actinides (MA), zirconium (Zr), molybdenum (Mo), fission products, etc.) of the metallic fuel slug and components (iron (Fe), chromium (Cr), tungsten (W), Mo, vanadium (V), niobium (Nb), etc.) of the stainless steel cladding tube by diffusion. Thus, the maximum allowable burnup and the maximum allowable operating temperature of the metallic fuel for fast reactors may be limited [J. Nucl. Mater., 204 (1993) p. 244-251 and J. Nucl. Mater., 204 (1993) p. 141-147].
Also, a diffusion couple experiment performed at 923 K by T. Ogata et al. demonstrated the occurrence of a reaction due to the interdiffusion between a metallic fuel slug and a cladding tube, and reported that the thickness of an interaction layer increased proportional to the reaction time [J. Nucl. Mater., 250 (1997) p. 171-175].
In order to prevent the interdiffusion reaction, General Electric (GE) disclosed a technique for inhibiting the interaction between a metallic fuel slug and a cladding tube by inserting an about 50 μm thick liner or sleeve formed of a metal of Zr, titanium (Ti), Nb, and Mo between the metallic fuel slug and the cladding tube.
Since the technique of GE essentially requires the introduction of an additional process, the production of the nuclear fuel rod may not only be complicated, but considerable additional costs may also be required.
Also, in order to remove quartz tube mold waste generated during the preparation of a fuel slug for fast reactors and simultaneously, to inhibit a fuel-cladding chemical interaction (FCCI) between metallic fuel slug and cladding tube, D. C. Crawford et al. melt-casted an about 200 μm thick zirconium tube and reported the results of their experiments. However, cracks may occur in the zirconium tube.
Metallic fuel for reactors has been considered important as a nuclear fuel of sodium-cooled fast reactors, an advanced nuclear fuel, due to high thermal conductivity and high nuclear proliferation resistance in conjunction with pyroprocessing. However, with respect to the metallic fuel, since metallic uranium as a fuel material and a fuel cladding material interdiffuse and react above 650° C., i.e., an operating temperature of the reactor, the thickness of a cladding tube decreases according to the operating time. As a result, the lifetime of the cladding tube may decrease due to the deterioration of the soundness thereof. In order to prevent the interaction phenomenon and improve the performance of the cladding material, research into using a material for preventing the interdiffusion and reaction between the fuel and the cladding tube has been conducted.
In Patent Document 1 (Korean Patent Application Laid-Open Publication No. KR-2009-0018396), a nuclear fuel rod for fast reactors, in which an oxide coating layer is formed on the inside of a cladding tube, is suggested in order to inhibit the fuel-cladding material interaction. Specifically, a concept of attaching chromium oxide, vanadium oxide, and zirconium oxide to the inside of the cladding tube by using an acid dissolution and oxidation method, a high-temperature oxidation method, an electrolytic oxidation method, and a vapor deposition method is suggested.
In Patent Document 2 (Korean Patent Application Laid-Open Publication No. KR-2010-0114392), a concept of depositing functional materials, such as titanium, nickel, chromium, vanadium, and zirconium, in multilayers is suggested in order to inhibit the fuel-cladding material interaction and improve the performance of the fuel cladding tube.
In Patent Document 3 (Korean Patent Application Laid-Open Publication No. KR-2010-0081961), a method of uniformly plating an inner wall of a fuel cladding tube and a concept of forming a nitride layer on a surface of the plating layer through an additional process of a nitridation treatment are suggested.
In Patent Document 4 (Japanese Patent Application Laid-Open Publication No. 2012-237574), a typical main body that may accommodate nuclear fuel and is formed of an iron-based material; and a cladding tube including an inner layer part composed of a carbon-based material that is formed on an inner circumferential surface of the main body and a reactor including the cladding tube are suggested in order to provide a cladding tube that may improve high-temperature characteristics and power generation efficiency, and a reactor including the cladding tube.
However, the fuel cladding tube for fast reactors is a seamless tube having a diameter of 7 mm, a thickness of 0.6 mm, and a length of 3,000 mm. Thus, there may be limitations in attaching the functional material for preventing interdiffusion to the inside of the thin and long tube, and treatment costs may be high.
Accordingly, the present inventors found that the interdiffusion between a metallic fuel slug and a cladding tube may be prevented by stabilizing components of the metallic fuel slug and fission products or impurities though the simple and uniform formation of an oxide layer, a nitride layer, or a carbide layer on the surface of the metallic fuel slug, thereby leading to completion of the present invention.